This invention relates to a processing apparatus for treating a solid surface using ions and, more particularly, to an apparatus for etching a solid surface by the use of ions, radical atoms and/or radical molecules in a plasma.
Recently, dry etching with employs an ion beam or ions, radical atoms and radical molecules in a plasma has been increasingly utilized in technical fields, such as the field of semiconductors, which require a very fine etching technique of several microns or less. As compared with the former method employing an ion beam, the latter method employing ions in a plasma has a higher ion density and has the merit of a higher etching rate, so that various studies have been made thereon. Among them, an expedient in which the plasma is generated by a microwave discharge is especially advantageous for several reasons, e.g. (1) since a discharge gas can break down even under a low pressure [below 1 .times. 10.sup.-3 Torr], the directions of ions become regular, (2) a plasma of especially high density can be generated, and (3) owing to an electrodeless discharge, the life of the apparatus is long, a chemical-active gas being usable. The details of this expedient are described in Japanese patent laid open specification No. 51-71597.
FIG. 1 shows the construction of a prior-art etching apparatus exploiting the microwave discharge. Microwaves produced by a microwave generator 1 are radiated into a rectangular waveguide 2. At a position of about 1/4 of a guide wavelength (.lambda..sub.g) from the terminal end of the rectangular waveguide 2, one end of an inner conductor 4 of a coaxial waveguide 3 protrudes into the waveguide 2, as shown in the figure. Through an antenna thus constructed, microwaves are propagated from the waveguide 2 to the coaxial waveguide 3. Further, an electric field of microwaves is propagated into a discharge area 5 through an insulator 7 as well as a device 6 for the coupling between the microwaves and the plasma. Shown at 9 is magnetic field supplying means, which produces an external magnetic field perpendicular to the aforecited microwave electric field. The magnetic field gives rise to a microwave discharge jointly with the microwave electric field. In general, the magnetic field to be produced by the magnetic field supplying means is a mirror type magnetic field which contains the plasma effectively. Numeral 8 designates a gas supply conduit, from which a gas such as N.sub.2, Ar and O.sub.2 under 1 .times. 10.sup.-3 to 10.sup.-5 Torr is introduced. The gas is ionized by the microwave discharge. Shown at 12 is a substrate to be etched, which is maintained at a predetermined potential by a power source 13 or is grounded. Owing to such a construction, an ion sheath is formed between the substrate 12 and the plasma generated by the microwave discharge in the magnetic field. The ions of the introduced gas pass through the ion sheath, and impinge on the substrate 12 to etch it.
However, when the coaxial waveguide is employed as in this construction, the inner conductor of the coaxial waveguide is held by the insulator and is therefore thermally isolated from the other parts, so that the inner conductor easily reaches a high temperature and is subject to thermal destruction at the connecting portion with the insulator. Moreover, in plasma etching apparatus the homogeneity of the plasma density distribution in the radial direction must be good. In the construction employing the coaxial waveguide, however, a part underneath the inner conductor or the microwave - plasma coupling device and a part surrounding the part exhibit large difference in plasma density, so that uniform etching cannot be carried out.